The present invention relates to lifting magnets and, more particularly, to a control system for selectively energizing and demagnetizing a lifting magnet.
As is well understood in the art, residual magnetism in the core of a lifting magnet and in a magnetically permeable load held by the magnet may make it impossible to release the load merely by deenergizing the lifting magnet. Accordingly, various systmes have been developed for obtaining a clean release of a held load by applying, to the magnet coil, a reverse current which is of smaller magnitude than the current applied to achieve lifting action. Such demagnetization is particularly necessary when lifting mixed loads such as scrap metal or when moving stacks of plates. With stacks of multiple plates, deenergizing the lift coil may release most of the load but the top plates may stick on the magnet, held by the residual magnetism.
In accordance with the conventional wisdom in the art, the necessary reverse current is applied to the coil lifting magnet by means of a reversing switch or contactors in the d.c. supply leads to the magnet. Operation of the contactors both broke the lift current and, through reversed connections, applied the reverse or drop current, this latter current being applied through a resistance appropriate for limiting the reverse current to the desired value. This system of interconnection has been conventional whether the basic supply to the magnet was a d.c. source such as rechargeable batteries or whether there was a bridge rectifier deriving direct current from a pair of a.c. supply leads or a generator.
As will be appreciated by those having practical experience, the switching of a heavy d.c. current to an electrical load having substantial inductance, as does a lifting magnet, produces severe arcing at the switch contacts. Thus the contactors required for such a current reversing operation have to be of exceptionally heavy and durable construction in order to survive myriad cycles of operation.
Contrary to this conventional wisdom, the present invention provides separate lift and demagnetizing coils in the magnet structure itself, each coil being provided with its own bridge rectifier means so that all switching can be accomplished on the a.c. supply leads.